Increasing conspecific density weakens the ability of intermediate predators to develop induced morphological defences to top predators

Summary Intraguild predation is common in nature, but it is unclear how species that both compete and eat each other can persist together. One possibility is that intermediate predators possess inducible morphological defences that protect them from top predators while not compromising their ability to compete with top predators. The ability of intermediate predators to develop morphological defences may be compromised in environments with a high density of conspecifics because of reduced resource availability and predation risk due to the saturating functional response of top predators. Furthermore, since morphological defences take time to develop, the type and extent of morphological defences may vary during development. We conducted an experiment to measure the phenotypic responses of an intermediate predator (larvae of the salamanderAmbystoma opacum) to the presence of a caged top predator (larvae of the dragonflyAnaxspp.) throughout ontological development in environments that differed in the density of conspecifics present. We also assessed how intermediate predators, reared in the different environments, differed in their vulnerability to top predators and ability to deplete their food resources. We found thatAnaxinduced morphological defences inA. opacum, but the extent of morphological change declined with the density of conspecifics. Moreover, some morphological traits disappeared, while others appeared just prior toA. opacummetamorphosis. The change inA. opacumphenotype in response toAnaxmadeA. opacumless vulnerable to predation byAnaxbut had no significant effect on the foraging ability ofA. opacum. Our study demonstrates that top predators can induce phenotypes in intermediate predators that reduce their vulnerability to top predators while not compromising their ability to feed on a common prey. An increase in intermediate predator density, however, could diminish the ability of intermediate predators to develop the full suite of morphological defences. The inability to develop the full suite of morphological defences may reduce the probability of persistence with top predators.